On the internal pollution mechanisms in the globular cluster NGC 6121(M4): Heavy-element abundances and AGB models

V. D'Orazi*, S. W. Campbell, M. Lugaro, J. C. Lattanzio, M. Pignatari, E. Carretta

*Corresponding author for this work

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    Globular clusters display significant variations in their light-element content, pointing to the existence of a second stellar generation formed from the ejecta of an earlier generation. The nature of these internal polluters is still a matter of debate: the two most popular scenarios indicate intermediate-mass asymptotic giant branch (IM-AGB) stars and fast rotating massive stars. Abundances determination for some key elements can help distinguish between these competitor candidates. We present in this paper Y abundances for a sample of 103 red giant branch stars in NGC 6121. Within measurement errors, we find that the [Y/Fe] is constant in this cluster contrary to a recent suggestion. For a subsample of six stars we also find [Rb/Fe] to be constant, consistent with previous studies showing no variation in other s-process elements. We also present a new set of stellar yields for IM-AGB stellar models of 5 and 6M⊙, including heavy element s-process abundances. The uncertainties on the mass-loss rate, the mixing length parameter and the nuclear reaction rates have a major impact on the stellar abundances. Within the IM-AGB pollution scenario, the constant abundance of heavy elements inside the cluster requires a marginal s-process efficiency in IM-AGB stars. Such a constrain could still be satisfied by the present models assuming a stronger mass-loss rate. The uncertainties mentioned above are limiting the predictive power of IM-AGB models. For these reasons, at the moment we are not able to clearly rule out their role as main polluters of the second population stars in globular clusters.

    Original languageEnglish
    Pages (from-to)366-381
    Number of pages16
    JournalMonthly Notices of the Royal Astronomical Society
    Issue number1
    Publication statusPublished - Jul 2013

    Bibliographical note

    This article has been accepted for publication in Monthly notices of the Royal Astronomical Society. Copyright 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.


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